Gravitational Waves from Extreme-Mass-Ratio Systems in Astrophysical Environments.

We establish a generic, fully relativistic formalism to study gravitational-wave emission by extreme-mass-ratio systems in spherically symmetric, nonvacuum black hole spacetimes. The potential applications to astrophysical setups range from black holes accreting baryonic matter to those within axionic clouds and dark matter environments, allowing one to assess the impact of the galactic potential, of accretion, gravitational drag, and halo feedback on the generation and propagation of gravitational waves. We apply our methods to a black hole within a halo of matter.

Gravitational Wave Signatures of Black Hole Quasinormal Mode Instability.

Black hole (BH) spectroscopy has emerged as a powerful approach to extracting spacetime information from gravitational wave (GW) observed signals. Yet, quasinormal mode (QNM) spectral instability under small scale perturbations has been recently shown to be a common classical general relativistic phenomenon [J. L.

Destabilizing the Fundamental Mode of Black Holes: The Elephant and the Flea.

Recent work applying the notion of pseudospectrum to gravitational physics showed that the quasinormal mode spectrum of black holes is unstable, with the possible exception of the longest-lived (fundamental) mode. The fundamental mode dominates the expected signal in gravitational wave astronomy, and there is no reason why it should have privileged status. We compute the quasinormal mode spectrum of two model problems where the Schwarzschild potential is perturbed by a small "bump" consisting of either a Pöschl-Teller potential or a Gaussian, and we show that the fundamental mode is destabilized under generic perturbations.

Understanding the "antikick" in the merger of binary black holes.

The generation of a large recoil velocity from the inspiral and merger of binary black holes represents one of the most exciting results of numerical-relativity calculations. While many aspects of this process have been investigated and explained, the "antikick," namely, the sudden deceleration after the merger, has not yet found a simple explanation. We show that the antikick can be understood in terms of the radiation from a deformed black hole where the anisotropic curvature distribution on the horizon correlates with the direction and intensity of the recoil.